Study on Carbon Fixation Ratio and Properties of Foamed Concrete

Using solid waste to sequester carbon dioxide not only reduces the greenhouse effect but also reuses resources. However, the existing solidified carbon dioxide storage materials are expensive and have poor storage effect. Therefore, in this study, cement, solid waste base material, and 30% hydrogen peroxide were used to make foamed concrete materials through chemical foaming, and XRD, BET, SEM, and thermogravimetric techniques were used to explore the amount of carbon dioxide adsorbed by foamed concrete materials under different ratio conditions. The results show that (1) the hydration products of the cementified materials mainly include C-S-H, Ht and Ca(OH)2, which are important factors for the storage of CO2. (2) A water–cement ratio of 0.7 and a foaming agent dosage of 10% are the best ratios for foamed concrete materials. With the increase of the water–cement ratio and the dosage of the foaming agent, the amount of CO2-sealed stock first increases and then decreases. (3) The maximum carbon dioxide sealing capacity of foamed concrete material is 66.35 kg/m3.


Introduction
Climate change is a great challenge related to human survival and long-term development. Low-carbon development has increasingly become one of the keys to sustainable human development. Therefore, how to reduce the CO 2 in the atmosphere and overcome the global greenhouse effect has attracted great attention globally. CO 2 capture is an effective method to reduce the CO 2 in the atmosphere. Many capture methods, such as absorption [1], adsorption [2], membrane separation [3], hydrate-based separation [4], and cryogenic distillation [5], have been developed. The captured CO 2 can further be reutilized in many fields, such as in the production of ammonia and urea, food and beverages, refrigerants, and fire extinguishing gases [6,7]. However, the reutilized CO 2 only accounts for 2% of the emissions [8], which is a tiny contribution to CO 2 reduction. In addition, most methods still exhibit shortcomings in terms of application convenience and cost. The cement-based materials can capture CO 2 from the atmosphere during the hydration reaction [9,10], and thus their application potentials in CO 2 sequestration have gradually attracted great attention [11][12][13]. Pure cement materials, such as cement and concrete, can sequester CO 2 but not cost-efficiently. Therefore, the composition and preparation process of cement material needs to be optimized for the economic cost of the implementation and carbon footprint issue [14]. The CO 2 diffusion rate is the controlling step for the early carbonation reaction rate of cement material [15,16]. Carbonation reduces the porosity of the carbonized area and forms carbonates at the edges of the cement particles, which inhibits CO 2 diffusion. Due to the compact structure of concrete, the carbonation depth is small. Therefore, only a very small amount of the components reacts with CO 2 . Figure 1 shows the experimental flow chart. Firstly, different foam concrete materials were prepared by varying the water-cement ratio and the amount of foaming agent. The materials were cured for 24 h naturally or by CO 2 , demolded, dried to terminate hydration, ground, and characterized by TGA, XRD, BET, and SEM. CO 2 sequestration was calculated from the TGA results to determine the optimal system. The hydration products, pore structure, specific surface area, and pore volume were analyzed by XRD, BET, and SEM to explore the influence mechanism of water-cement ratio and forming agent dosage on CO 2 sequestration.

Experimental Design
Materials 2023, 16, x FOR PEER REVIEW 3 of 16 different systems on CO2 sequestration, aimed to provide guidance for the utilization of solid waste materials and mitigation of the greenhouse effect. Figure 1 shows the experimental flow chart. Firstly, different foam concrete materials were prepared by varying the water-cement ratio and the amount of foaming agent. The materials were cured for 24 h naturally or by CO2, demolded, dried to terminate hydration, ground, and characterized by TGA, XRD, BET, and SEM. CO2 sequestration was calculated from the TGA results to determine the optimal system. The hydration products, pore structure, specific surface area, and pore volume were analyzed by XRD, BET, and SEM to explore the influence mechanism of water-cement ratio and forming agent dosage on CO2 sequestration.

Preparation of Foam Concrete Materials
The cementitious material was prepared with 50% PO cement, 35% slag, and 15% fly ash. Hydrogen peroxide (30%) [32], sodium stearate, and calcium oxide were used as the foaming agent, foam stabilizer, and alkali activator, respectively. Table 1 lists the composition of each system. The solid waste-based porous materials were obtained from the systems by chemical foaming. First, pour the pre-weighed cementing material and foaming agent dry powder material into the mixing pot for 1 min at low speed, and then pour the water at normal

Preparation of Foam Concrete Materials
The cementitious material was prepared with 50% PO cement, 35% slag, and 15% fly ash. Hydrogen peroxide (30%) [32], sodium stearate, and calcium oxide were used as the foaming agent, foam stabilizer, and alkali activator, respectively. Table 1 lists the composition of each system. The solid waste-based porous materials were obtained from the systems by chemical foaming. First, pour the pre-weighed cementing material and foaming agent dry powder material into the mixing pot for 1 min at low speed, and then pour the water at normal temperature into the mixing pot for 1 min at low speed according to the experimental requirements, add the CaO alkali activator and stir for 2 min. Finally, pour H 2 O 2 into the mixing pot and mix with cement slag slurry, and quickly stir for 10s to prepare foamed concrete. After that, quickly pour it into the prepared 5 × 5 × 5 cm mold. After the foaming is finished, cover the surface with a layer of plastic wrap to prevent water evaporation. After standing for 1 day, remove the mold, wrap the sample in a plastic bag, and send it to the curing box for curing until the specified age (3 days). The actual picture of material preparation is shown in Figure 2.
temperature into the mixing pot for 1 min at low speed according to the experimental requirements, add the CaO alkali activator and stir for 2 min. Finally, pour H2O2 into the mixing pot and mix with cement slag slurry, and quickly stir for 10s to prepare foamed concrete. After that, quickly pour it into the prepared 5 × 5 × 5 cm mold. After the foaming is finished, cover the surface with a layer of plastic wrap to prevent water evaporation. After standing for 1 day, remove the mold, wrap the sample in a plastic bag, and send it to the curing box for curing until the specified age (3 days). The actual picture of material preparation is shown in Figure 2.

CO2 Curing and Sequestration
The CO2 sequestration experiment simulated the real CO2 adsorption process in foam concrete. First, the curing chamber was purged with pure CO2 (99.9%) for a period of time to remove the air. The samples were placed in the curing chamber in the order of labels, and 0.1 MPa CO2 was continuously introduced for 24 h for CO2 curing and sequestration.

Characterizations
The hydration product composition of the cementitious material was determined by X-ray diffraction (XRD) using a Rigaku Ultima IV X-ray diffractometer in the scanning range of 5-80° at the scanning speed of 8°/min and the step size of 0.02°.
The evolution of the multiscale porous structure of the solid waste-based porous material with the variation of water-cement ratio and foaming agent dose was characterized by SEM and BET analysis. Specifically, the microstructure and surface morphology were imaged with a Nova Nano SEM 450 high-resolution scanning electron microscope, and the specific surface area and average pore diameter were measured with a fully automated ASAP-2460 surface area and porosity analyzer.
TGA was conducted on the prepared solid waste-based material samples at the heating rate of 10 °C/min under a nitrogen atmosphere. The pyrolysis process was analyzed with TGA and DTG curves, and CO2 sequestration was calculated from the weight loss and dry density of the sample.

Effects of Water-Cement Ratio on CO2 Sequestration
The naturally cured samples at the water-cement ratios of 0.4, 0.7, and 0.8 are denoted as NC 0.4, NC 0.7, and NC 0.8, respectively, and the corresponding carbon-cured

CO 2 Curing and Sequestration
The CO 2 sequestration experiment simulated the real CO 2 adsorption process in foam concrete. First, the curing chamber was purged with pure CO 2 (99.9%) for a period of time to remove the air. The samples were placed in the curing chamber in the order of labels, and 0.1 MPa CO 2 was continuously introduced for 24 h for CO 2 curing and sequestration.

Characterizations
The hydration product composition of the cementitious material was determined by X-ray diffraction (XRD) using a Rigaku Ultima IV X-ray diffractometer in the scanning range of 5-80 • at the scanning speed of 8 • /min and the step size of 0.02 • .
The evolution of the multiscale porous structure of the solid waste-based porous material with the variation of water-cement ratio and foaming agent dose was characterized by SEM and BET analysis. Specifically, the microstructure and surface morphology were imaged with a Nova Nano SEM 450 high-resolution scanning electron microscope, and the specific surface area and average pore diameter were measured with a fully automated ASAP-2460 surface area and porosity analyzer.
TGA was conducted on the prepared solid waste-based material samples at the heating rate of 10 • C/min under a nitrogen atmosphere. The pyrolysis process was analyzed with TGA and DTG curves, and CO 2 sequestration was calculated from the weight loss and dry density of the sample.

Effects of Water-Cement Ratio on CO 2 Sequestration
The naturally cured samples at the water-cement ratios of 0.4, 0.7, and 0.8 are denoted as NC 0.4, NC 0.7, and NC 0.8, respectively, and the corresponding carbon-cured samples are denoted as CC 0.4, CC 0.7, and CC 0.8, respectively. The hydration products, specific surface areas, average pore sizes, and CO 2 sequestrations of these samples were analyzed and characterized to determine the optimal water-cement ratio. Figure 3 shows the XRD patterns of the naturally cured samples with different watercement ratios. The diffraction peak at 2θ = 29.5 • is the strongest, which is the characteristic peak of the main hydration product C-S-H, and the peak intensity increases with the increase of water-cement ratio. The diffraction peaks at 2θ = 36.9 • , 47.1 • , and 50.7 • are attributed to Ca(OH) 2 . These diffraction peaks in NC 0.4 are not obvious, possibly because the low water-cement ratio causes incomplete hydration or low Ca(OH) 2 content, which will eventually result in low CO 2 sequestrations. The characteristic diffraction peaks of calcite are detected at 2θ = 29.4 • , 39.4 • , and 42.8 • . The cementitious material contains calcite. CaCO 3 can also be formed by the carbonization of Ca 2+ during the material preparation. The diffraction peaks at 2θ = 25.7 • , 26.6 • , 32.1 • , and 43.9 • can be indexed to vaterite, and the peaks also become stronger with the increase of water-cement ratio. The diffraction peaks at 2θ = 10.9 • and 23.3 • can be assigned to the Ht phase, and that at 2θ = 30.8 • is due to the mayenite phase. The comparison of the XRD patterns of samples with different water-cement ratios suggest they contain the same phases, indicating that changing the water-cement ratio will not alter the types of hydration products.

XRD Analysis
samples are denoted as CC 0.4, CC 0.7, and CC 0.8, respectively. The hydration products, specific surface areas, average pore sizes, and CO2 sequestrations of these samples were analyzed and characterized to determine the optimal water-cement ratio.
3.1.1. XRD Analysis Figure 3 shows the XRD patterns of the naturally cured samples with different watercement ratios. The diffraction peak at 2θ = 29.5° is the strongest, which is the characteristic peak of the main hydration product C-S-H, and the peak intensity increases with the increase of water-cement ratio. The diffraction peaks at 2θ = 36.9°, 47.1°, and 50.7° are attributed to Ca(OH)2. These diffraction peaks in NC 0.4 are not obvious, possibly because the low water-cement ratio causes incomplete hydration or low Ca(OH)2 content, which will eventually result in low CO2 sequestrations. The characteristic diffraction peaks of calcite are detected at 2θ = 29.4°, 39.4°, and 42.8°. The cementitious material contains calcite. CaCO3 can also be formed by the carbonization of Ca 2+ during the material preparation. The diffraction peaks at 2θ = 25.7°, 26.6°, 32.1°, and 43.9° can be indexed to vaterite, and the peaks also become stronger with the increase of water-cement ratio. The diffraction peaks at 2θ = 10.9° and 23.3° can be assigned to the Ht phase, and that at 2θ = 30.8° is due to the mayenite phase. The comparison of the XRD patterns of samples with different water-cement ratios suggest they contain the same phases, indicating that changing the water-cement ratio will not alter the types of hydration products.  Figure 4 shows the SEM images of the samples with different water-cement ratios. At the low water-cement ratio of 0.4, the samples exhibit dense surfaces, high dry densities, significantly ununiform pore sizes, wide size distributions, and small specific surface areas (Figure 4a1,a2). As the water-cement ratio increases, the sample surface becomes loose, the pores on the surface are refined, the dry density decreases, the number of pores in the sample increase, and pore sizes are more uniform (Figure 4b1,b2,c1,c2).  Figure 4 shows the SEM images of the samples with different water-cement ratios. At the low water-cement ratio of 0.4, the samples exhibit dense surfaces, high dry densities, significantly ununiform pore sizes, wide size distributions, and small specific surface areas (Figure 4a1,a2). As the water-cement ratio increases, the sample surface becomes loose, the pores on the surface are refined, the dry density decreases, the number of pores in the sample increase, and pore sizes are more uniform (Figure 4b1,b2,c1,c2). Connected pores are clearly observed in NC 0.7 and CC 0.7 (Figure 4b1,b2), which are conducive to CO 2 diffusion, increase the contact area between CO 2 and foam concrete material and eventually improve CO 2 sequestration. Their high magnification images reveal the fibrous hydration product C-S-H gel and the flaky Ca(OH) 2 in NC0.7. However, there is much more CaCO 3 in CC 0.7, and Ca(OH) 2 flakes are barely seen (Figure 4d1,d2), suggesting most of the Ca(OH) 2 is carbonated into CaCO 3 .

SEM Analysis
Connected pores are clearly observed in NC 0.7 and CC 0.7 (Figure 4b1,b2), wh conducive to CO2 diffusion, increase the contact area between CO2 and foam co material and eventually improve CO2 sequestration. Their high magnification i reveal the fibrous hydration product C-S-H gel and the flaky Ca(OH)2 in NC0.7. Ho there is much more CaCO3 in CC 0.7, and Ca(OH)2 flakes are barely seen (Figure 4d suggesting most of the Ca(OH)2 is carbonated into CaCO3. Figure 4. SEM images of samples with different water-cement ratios before and after carbon and high magnification images of the samples with water-cement ratio of 0.7 (a1) waterratio 0.4, non-carbonized sample with 10% foaming agent; (a2) water-cement ratio 0.4, carb sample with 10% foaming agent; (b1) water-cement ratio 0.7, non-carbonized sample wi foaming agent; (b2) water-cement ratio 0.7, carbonized sample with 10% foaming agent; (c1) cement ratio 0.8; For non-carbonized sample with 10% foaming agent, (c2) water-cement ra and carbonized sample with 10% foaming agent, (d1) water-cement ratio 0.7 at 5000 magnification, and (d2) sample with 10% foaming agent after carbonization at 5000 magnification.

BET Analysis
The samples NC 0.4, NC 0.7, NC 0.8, CC 0.4, CC 0.7, and CC 0.8 wer characterized for N2 adsorption/desorption isotherm and pore size distribution. As in Figure 5a, the nitrogen adsorption of all samples increases gradually up to a r pressure of 0.5 and then increases dramatically as the relative pressure rises furthe . SEM images of samples with different water-cement ratios before and after carbonization and high magnification images of the samples with water-cement ratio of 0.7 (a1) water-cement ratio 0.4, non-carbonized sample with 10% foaming agent; (a2) water-cement ratio 0.4, carbonized sample with 10% foaming agent; (b1) water-cement ratio 0.7, non-carbonized sample with 10% foaming agent; (b2) water-cement ratio 0.7, carbonized sample with 10% foaming agent; (c1) water-cement ratio 0.8; For non-carbonized sample with 10% foaming agent, (c2) water-cement ratio 0.8, and carbonized sample with 10% foaming agent, (d1) water-cement ratio 0.7 at 5000 times magnification, and (d2) sample with 10% foaming agent after carbonization at 5000 times magnification.

BET Analysis
The samples NC 0.4, NC 0.7, NC 0.8, CC 0.4, CC 0.7, and CC 0.8 were then characterized for N 2 adsorption/desorption isotherm and pore size distribution. As shown in Figure 5a, the nitrogen adsorption of all samples increases gradually up to a relative pressure of 0.5 and then increases dramatically as the relative pressure rises further. This pattern of nitrogen adsorption is due to the capillary condensation phenomenon, which is caused by the slit-like pore structure. The adsorption capacity is extremely high as the relative pressure is close to 1, approaching the saturation adsorption due to multi-layer adsorption. The N 2 adsorption capacities before and after carbonization are different because the precipitates generated by the CO 2 absorption block the mesopores in foam concrete and hinder the adsorption.
concrete and hinder the adsorption. Figure 5b shows the pore size distributions of these samples, and Figure 6 compares their specific surface areas and pore structures. As can be seen, the pore sizes of all of them mainly fall between 2 and 15 nm, with some exceptions between 15 and 60 nm, indicating that the pores in the samples are mainly mesopores. Before carbonization, both pore volume and specific surface area gradually increase with the increase of water-cement ratio and reach the highest values of 0.144 cm 3 /g and 25.412 m 2 /g, respectively, at the ratio of 0.7. The average pore size shows similar changes, and the value at the water-cement ratio of 0.7 is 12.84 nm, second only to that of NC 0.8. Overall, CO2 diffusion is easier in NC 0.7. Its large surface area provides a large contact surface with CO2, which also improves CO2 sequestration.

CO2 Sequestration
To determine the CO2 sequestration amount, TGA was conducted on the naturally cured and carbon-cured samples with different water-cement ratios. Figure 7 shows their  Figure 5b shows the pore size distributions of these samples, and Figure 6 compares their specific surface areas and pore structures. As can be seen, the pore sizes of all of them mainly fall between 2 and 15 nm, with some exceptions between 15 and 60 nm, indicating that the pores in the samples are mainly mesopores. Before carbonization, both pore volume and specific surface area gradually increase with the increase of water-cement ratio and reach the highest values of 0.144 cm 3 /g and 25.412 m 2 /g, respectively, at the ratio of 0.7. The average pore size shows similar changes, and the value at the water-cement ratio of 0.7 is 12.84 nm, second only to that of NC 0.8. Overall, CO 2 diffusion is easier in NC 0.7. Its large surface area provides a large contact surface with CO 2 , which also improves CO 2 sequestration. pattern of nitrogen adsorption is due to the capillary condensation phenomenon, which is caused by the slit-like pore structure. The adsorption capacity is extremely high as the relative pressure is close to 1, approaching the saturation adsorption due to multi-layer adsorption. The N2 adsorption capacities before and after carbonization are different because the precipitates generated by the CO2 absorption block the mesopores in foam concrete and hinder the adsorption. Figure 5b shows the pore size distributions of these samples, and Figure 6 compares their specific surface areas and pore structures. As can be seen, the pore sizes of all of them mainly fall between 2 and 15 nm, with some exceptions between 15 and 60 nm, indicating that the pores in the samples are mainly mesopores. Before carbonization, both pore volume and specific surface area gradually increase with the increase of water-cement ratio and reach the highest values of 0.144 cm 3 /g and 25.412 m 2 /g, respectively, at the ratio of 0.7. The average pore size shows similar changes, and the value at the water-cement ratio of 0.7 is 12.84 nm, second only to that of NC 0.8. Overall, CO2 diffusion is easier in NC 0.7. Its large surface area provides a large contact surface with CO2, which also improves CO2 sequestration.

CO2 Sequestration
To determine the CO2 sequestration amount, TGA was conducted on the naturally cured and carbon-cured samples with different water-cement ratios. Figure 7 shows their

CO 2 Sequestration
To determine the CO 2 sequestration amount, TGA was conducted on the naturally cured and carbon-cured samples with different water-cement ratios. Figure 7 shows their TGA and DTG curves. All samples show constant weight losses during the whole heating process. The weight loss between 30 • C and 100 • C is mainly caused by the evaporation of adsorbed water, and that from 100-200 • C is mainly due to the decomposition of the hydration product C-S-H. The decomposition of the hydration product Ht phase results in the weight loss between 200 • C and 350 • C. The weight loss between 350 • C and 500 • C is mainly caused by the loss of Ca(OH) 2 , and that between 500 • C and 900 • C is mainly attributed to the decomposition of carbonate. The carbonate is mainly from the material itself and the carbonization of Ca(OH) 2 . Therefore, CO 2 sequestration can be obtained as the difference in the weight loss between naturally cured and carbon-cured samples during the carbonate decomposition.

Effects of Foaming Agent Dosage on CO2 Sequestration
The study on the effects of water-cement ratio on CO2 sequestration suggests that the optimal water-cement ratio is 0.7. Therefore, the effects of the foaming agent dosage on CO2 sequestration were investigated with the water-cement ratio fixed at 0.7 and foaming agent dose varied to 8%, 10%, and 12%, respectively. The corresponding naturally cured samples are denoted as NC 8%, NC 10%, and NC 12%, and the corresponding carboncured samples are denoted as CC 8%, CC 10%, and CC 12%. The hydration products, specific surface areas, average pore sizes, and CO2 sequestrations of these samples were analyzed and characterized to determine the optimal foaming agent dosage. Figure 8 shows the XRD patterns of the samples prepared at different foaming agent doses. As can be seen, the changes in the XRD pattern with the foaming agent dosage are similar to those with the change in water-cement ratio. The diffraction peak at 2θ = 29.5° is the strongest one, which is the characteristic diffraction peak of the main hydration product C-S-H of the cementitious material. The intensity of the diffraction peak increases with the increase of the foaming agent dosage. The diffraction peaks at 2θ = 36.1°, 34.1°, 47.1°, and 52.2° are indexed to Ca(OH)2. These diffraction peaks also become stronger with the increase of foaming agent dosage. The diffraction peaks at 2θ = 10.9° and 23.3° are indexed to the Ht phase, which is a double-layer of metal hydroxides composed of Mg 2+ (or Ca 2+ ) and Al 3+ . Similarly, the diffraction peaks of CaCO3 at 2θ = 29.4°, 39.4°, and 43.2° are observed in the XRD patterns of all samples, suggesting that main phases remain unchanged with the increase of foaming agent dosage and the foaming agent dosage has no effects on the type of hydration products of the material. As can be seen from Figure 7a, in the naturally solidified sample, the weight loss rate of NC 0.8, NC 0.7, and NC 0.4 is 19.59%, 17.55%, and 18.75%, respectively, within the whole temperature range. As shown in Figure 7b, the weight loss rate of NC 0.4 is 4.62%, that of NC 0.7 is 4.03%, and that of NC 0.8 is 4.15% in the temperature range of 500-900 • C of carbonate decomposition, which is mainly the carbonation of Ca(OH) 2 in the material itself.

XRD Analysis
Specifically, it shows the highest weight losses from 30-100 • C and 200-350 • C, corresponding to the decomposition of the Ht phase (Figure 7b). The weight loss of NC 0.4 is the highest in the Ca(OH) 2 decomposition temperature range, indicating that its Ca(OH) 2 consumption is the least. The weight loss of NC 0.4 in the carbonate decomposition temperature range of 500-900 • C is higher than those of the other two naturally cured samples.
Among the carbon-cured samples, the weight loss of CC 0.7 is the highest, with values up to 28.25% (Figure 7c). The weight loss rate of CC 0.4 is 27.05%, and that of CC 0.8 is 26.62%. As shown in Figure 6d, the weight loss rate of CC 0.4 is 18.07%, and that of CC 0.7 is 19.39% in the 500-900 • C temperature range of carbonate decomposition. The weight loss rate of CC 0.8 is 19.87%, which is mainly due to the effective absorption of CO 2 by all hydration products in the carbonization process and their conversion into CaCO 3 -dominated carbonates.
Unlike the naturally cured sample, CC 0.7 shows the highest weight loss in the temperature range of 500-900 • C among its different weight loss temperature ranges. As can be seen from the DTG curves of the carbon-cured samples, they exhibit the same weight loss temperature ranges as the naturally cured samples; however, the weight losses in the decomposition temperature ranges of Ca(OH) 2 and Ht phase are lower, and the weight loss in the carbonate decomposition temperature range of 500-900 • C is much greater (Figure 7d). These results indicate that during CO 2 curing, the hydration products and CO 2 are converted into carbonates. The weight loss of carbonate increases first and then decreases with the increase of water-cement ratio, and so does the sequestration content of CO 2 , indicating that extremely high water-cement ratios are inconducive to CO 2 sequestration. The high dry density, dense structure, fewer pores, and ununiform pore sizes of the material with the water-cement ratio of 0.4 are unfavorable to CO 2 sequestration. At the water-cement ratio of 0.7, the strong hydration reaction of the cementitious material results in a loose structure, many uniform pores, and even connected pores, which can promote CO 2 diffusion between the pores and increase the contact area. Further increasing the water-cement ratio to 0.8 decreases CO 2 sequestration, which may be due to the good fluidity of the slurry, which easily breaks the bubbles and reduces the pore volume. Although more hydration products are generated, the reductions in CO 2 diffusion rate and contact area cause low sequestration.

Effects of Foaming Agent Dosage on CO 2 Sequestration
The study on the effects of water-cement ratio on CO 2 sequestration suggests that the optimal water-cement ratio is 0.7. Therefore, the effects of the foaming agent dosage on CO 2 sequestration were investigated with the water-cement ratio fixed at 0.7 and foaming agent dose varied to 8%, 10%, and 12%, respectively. The corresponding naturally cured samples are denoted as NC 8%, NC 10%, and NC 12%, and the corresponding carbon-cured samples are denoted as CC 8%, CC 10%, and CC 12%. The hydration products, specific surface areas, average pore sizes, and CO 2 sequestrations of these samples were analyzed and characterized to determine the optimal foaming agent dosage. Figure 8 shows the XRD patterns of the samples prepared at different foaming agent doses. As can be seen, the changes in the XRD pattern with the foaming agent dosage are similar to those with the change in water-cement ratio. The diffraction peak at 2θ = 29.5 • is the strongest one, which is the characteristic diffraction peak of the main hydration product C-S-H of the cementitious material. The intensity of the diffraction peak increases with the increase of the foaming agent dosage. The diffraction peaks at 2θ = 36.

SEM Analysis
The SEM imaging reveals that the foaming agent dosage affects the microscopic pore distribution in foam concrete significantly (Figure 9). At the foaming agent dose of 8%, more round pores are formed in the sample; however, pore sizes are ununiform. The low foaming agent dose also results in a low relative water content and dense structure. As the foaming agent dose increases to 10%, the relative water content and the degree of hydration of the cementitious material increase, and more bubbles are generated. There are more round pores between the particles of the cementitious material, and connected pores are clearly observed, which lowers the resistance to CO 2 diffusion, increases the contact area, and thus is conducive to CO 2 sequestration. However, further increasing the foaming agent dose to an excess amount, 12%, the number of bubbles increases, and the bubbles tend to break or connect with each other to form bigger bubbles, which leads to low dry density and thus is inconducive to CO 2 sequestration. It is worth noting that the hydration products C-S-H gel and Ca(OH) 2 flakes are also observed in the 5000× magnification image.

SEM Analysis
The SEM imaging reveals that the foaming agent dosage affects the microscopic pore distribution in foam concrete significantly (Figure 9). At the foaming agent dose of 8%, more round pores are formed in the sample; however, pore sizes are ununiform. The low foaming agent dose also results in a low relative water content and dense structure. As the foaming agent dose increases to 10%, the relative water content and the degree of hydration of the cementitious material increase, and more bubbles are generated. There are more round pores between the particles of the cementitious material, and connected pores are clearly observed, which lowers the resistance to CO2 diffusion, increases the contact area, and thus is conducive to CO2 sequestration. However, further increasing the foaming agent dose to an excess amount, 12%, the number of bubbles increases, and the bubbles tend to break or connect with each other to form bigger bubbles, which leads to low dry density and thus is inconducive to CO2 sequestration. It is worth noting that the hydration products C-S-H gel and Ca(OH)2 flakes are also observed in the 5000× magnification image.

BET Analysis
The pore structure of foam concrete is an important factor affecting CO 2 sequestration efficiency. In addition to water-cement ratio, foaming agent dosage is also an important factor affecting the pore structure. Therefore, NC 8%, NC 10%, NC 12%, CC 8%, CC 10%, and CC 12% were characterized by BET analysis for their pore properties. Figures 10 and 11 show the N 2 adsorption/desorption isotherms, pore size distributions, and specific surface areas of these samples. Similar to those obtained at different water-cement ratios, the N 2 adsorption/desorption isotherms of the samples prepared with different foaming agent dosages are the typical type IV isotherms (Figure 10a). The N 2 adsorption capacity increases with the increase of pressure, indicating that the pore structure is still dominated by mesopores, despite the variations in the foaming agent dosage.
The pore sizes range from 2 nm to 70 nm and mainly distribute in the range of 3-10 nm (Figure 10b). Although the specific surface area of NC10% is slightly smaller, its pore volume and average pore size are the largest, which is conducive to CO 2 diffusion and transport, improves the reaction efficiency between CO 2 and the hydration products or Ca 2+ , and thus increases CO 2 sequestration. Materials 2023, 16, x FOR PEER REVIEW Figure 9. SEM images of samples prepared with different foaming agent doses be carbonization and high magnification images of the samples prepared with 10% foam uncarbonized sample with a water-cement ratio of 0.7 and foaming agent of 8%, (a2) sample with a water-cement ratio of 0.7 and foaming agent of 8%, (b1) uncarbonized water-cement ratio of 0.7 and foaming agent of 10%, (b2) water-cement ratio of 0.7 agent of 10% and (c1) water-cement ratio of 0.7, For the uncarbonized sample with agent, (c2) water-cement ratio 0.7, and the carbonized sample with 12% foaming age cement ratio 0.7, and the uncarbonized sample with 10% foaming agent, and magnification of water-cement ratio 0.7, and the carbonized sample with 10% foamin

BET Analysis
The pore structure of foam concrete is an important factor affecting CO2 s efficiency. In addition to water-cement ratio, foaming agent dosage is also Figure 9. SEM images of samples prepared with different foaming agent doses before and after carbonization and high magnification images of the samples prepared with 10% foaming agent. (a1) uncarbonized sample with a water-cement ratio of 0.7 and foaming agent of 8%, (a2) uncarbonized sample with a water-cement ratio of 0.7 and foaming agent of 8%, (b1) uncarbonized sample with a water-cement ratio of 0.7 and foaming agent of 10%, (b2) water-cement ratio of 0.7 and foaming agent of 10% and (c1) water-cement ratio of 0.7, For the uncarbonized sample with 12% foaming agent, (c2) water-cement ratio 0.7, and the carbonized sample with 12% foaming agent, (d1) water-cement ratio 0.7, and the uncarbonized sample with 10% foaming agent, and (d2) 5000-fold magnification of water-cement ratio 0.7, and the carbonized sample with 10% foaming agent. by mesopores, despite the variations in the foaming agent dosage.
The pore sizes range from 2 nm to 70 nm and mainly distribute in the range of 3-10 nm (Figure 10b). Although the specific surface area of NC10% is slightly smaller, its pore volume and average pore size are the largest, which is conducive to CO2 diffusion and transport, improves the reaction efficiency between CO2 and the hydration products or Ca 2+ , and thus increases CO2 sequestration.

CO2 Sequestration
CO2 sequestration directly reflects the CO2 adsorption performance of a foam concrete material. Therefore, TGA analysis was also carried out on the foam concrete materials prepared with different foaming agent doses to calculate their CO2 sequestrations. As shown in Figure 12a,c, each sample shows obvious weight loss in the temperature range of 30-900 °C. The weight loss of NC12% is the highest among the naturally cured samples, with values of up to 23.8%. The weight loss rate of 10% NC is 17.55%, and that of 8% NC is 23.64. CC 10% has a maximum weight loss of 28.21%, CC 12% has a weight loss of 24.37, CC 8% has a weight loss of 23.98%, and that of CC 10% is the highest among the carbon-cured sample with values of up to 27.0%. DTG curves suggest that the weight loss caused by the decomposition of the Ht phase at 200-350 °C and that of Ca(OH)2 at 350-500 °C are lower in the carbonized sample than in the naturally cured sample (Figure 12d vs. Figure 12b). However, in the temperature range of 500-800 by mesopores, despite the variations in the foaming agent dosage.
The pore sizes range from 2 nm to 70 nm and mainly distribute in the range of 3-10 nm (Figure 10b). Although the specific surface area of NC10% is slightly smaller, its pore volume and average pore size are the largest, which is conducive to CO2 diffusion and transport, improves the reaction efficiency between CO2 and the hydration products or Ca 2+ , and thus increases CO2 sequestration.

CO2 Sequestration
CO2 sequestration directly reflects the CO2 adsorption performance of a foam concrete material. Therefore, TGA analysis was also carried out on the foam concrete materials prepared with different foaming agent doses to calculate their CO2 sequestrations. As shown in Figure 12a,c, each sample shows obvious weight loss in the temperature range of 30-900 °C. The weight loss of NC12% is the highest among the naturally cured samples, with values of up to 23.8%. The weight loss rate of 10% NC is 17.55%, and that of 8% NC is 23.64. CC 10% has a maximum weight loss of 28.21%, CC 12% has a weight loss of 24.37, CC 8% has a weight loss of 23.98%, and that of CC 10% is the highest among the carbon-cured sample with values of up to 27.0%. DTG curves suggest that the weight loss caused by the decomposition of the Ht phase at 200-350 °C and that of Ca(OH)2 at 350-500 °C are lower in the carbonized sample than in the naturally cured sample (Figure 12d vs. Figure 12b). However, in the temperature range of 500-800

CO 2 Sequestration
CO 2 sequestration directly reflects the CO 2 adsorption performance of a foam concrete material. Therefore, TGA analysis was also carried out on the foam concrete materials prepared with different foaming agent doses to calculate their CO 2 sequestrations. As shown in Figure 12a,c, each sample shows obvious weight loss in the temperature range of 30-900 • C. The weight loss of NC12% is the highest among the naturally cured samples, with values of up to 23.8%. The weight loss rate of 10% NC is 17.55%, and that of 8% NC is 23.64. CC 10% has a maximum weight loss of 28.21%, CC 12% has a weight loss of 24.37, CC 8% has a weight loss of 23.98%, and that of CC 10% is the highest among the carbon-cured sample with values of up to 27.0%. DTG curves suggest that the weight loss caused by the decomposition of the Ht phase at 200-350 • C and that of Ca(OH) 2 at 350-500 • C are lower in the carbonized sample than in the naturally cured sample (Figure 12d vs. Figure 12b). However, in the temperature range of 500-800 • C, the weight loss of the carbon-cured sample is higher, suggesting that the hydration products effectively absorb CO 2 and convert it into CaCO 3 . The weight loss caused by the decomposition of carbonate gradually increases with the increase of the foaming agent dose and then declines at the dose of 12%. This can be explained by the fact that the relative water content of the material increases with the increase of the foaming agent dose, and the hydration reaction of the material becomes more thorough. At the same time, the number of generated bubbles increases, which provides more adsorption sites for CO 2 sequestration. However, the extremely high amount of foaming agent tends to generate large pores. The good fluidity of the slurry also tends to break the foam in the concrete and thus reduces the pore volume, which leads to the reduction of the actual adsorption site for CO 2 and lowers CO 2 sequestration. °C, the weight loss of the carbon-cured sample is higher, suggesting that the hydratio products effectively absorb CO2 and convert it into CaCO3. The weight loss caused by th decomposition of carbonate gradually increases with the increase of the foaming agen dose and then declines at the dose of 12%. This can be explained by the fact that the relativ water content of the material increases with the increase of the foaming agent dose, an the hydration reaction of the material becomes more thorough. At the same time, th number of generated bubbles increases, which provides more adsorption sites for CO sequestration. However, the extremely high amount of foaming agent tends to generat large pores. The good fluidity of the slurry also tends to break the foam in the concret and thus reduces the pore volume, which leads to the reduction of the actual adsorptio site for CO2 and lowers CO2 sequestration.

Discussion
There have been many studies on the recovery, capture, and utilization of CO however, most methods have problems of poor storage effect and high cost. In this study industrial wastes such as cement, slag, and fly ash (in combination with hydroge peroxide) were used as chemical foaming agents in the production of foamed concrete This was performed using an alkali excitation agent, and the pore structure of the materia was adjusted by changing the water-cement ratio and the amount of foaming agent. I was found that C-S-H, Ca(OH)2, and other substances produced in the hydration proces of the material would precipitate carbonate with carbon dioxide. With the increase o water-cement ratio and dosage of foaming agent, thermogravimetric data showed tha the carbon dioxide sealing capacity showed a trend of first increasing and then decreasing It was found by BET and SEM that when the water-cement ratio increased to 0.7, and th dosage of foaming agent increased to 10%, the specific surface area of the materia

Discussion
There have been many studies on the recovery, capture, and utilization of CO 2 ; however, most methods have problems of poor storage effect and high cost. In this study, industrial wastes such as cement, slag, and fly ash (in combination with hydrogen peroxide) were used as chemical foaming agents in the production of foamed concrete. This was performed using an alkali excitation agent, and the pore structure of the material was adjusted by changing the water-cement ratio and the amount of foaming agent. It was found that C-S-H, Ca(OH) 2 , and other substances produced in the hydration process of the material would precipitate carbonate with carbon dioxide. With the increase of water-cement ratio and dosage of foaming agent, thermogravimetric data showed that the carbon dioxide sealing capacity showed a trend of first increasing and then decreasing. It was found by BET and SEM that when the water-cement ratio increased to 0.7, and the dosage of foaming agent increased to 10%, the specific surface area of the material gradually increased, the proportion of connected pores increased, and the size of pores gradually became uniform. The samples are all mesoporous, with pore sizes ranging from 2-50 nm. With the further increase of water-cement ratio and dosage of foaming agent, the sealing effect is poor because the foaming is too intense, the water-cement ratio is high, and bubbles burst easily, leading to the increase of the proportion of large pores. After CO 2 storage, the specific surface area and average pore diameter of the sample increased slightly, which could be caused by the hydration reaction of Ca 2+ on the surface of slag particles before carbonization. During the carbonization process, Ca 2+ was generated by continuous decalcification of C-S-H gel, and CO 2 was dissolved into CO 3 2− and HCO 3 − in the carbonization chamber. Ca 2+ combines with CO 3 2− in the solution to produce CaCO 3 with a stable structure. After decalcification, C-S-H gels undergo condensation reactions with each other and finally form calcium-modified gels with a higher degree of polymerization, forming deeper ion-leaching channels, thus increasing pore volume and average pore diameter.
This study explored the optimal water-cement ratio and amount of foaming agent. The amount of alkali activator and alkali excitation system are also important factors affecting the adsorption of carbon dioxide. In the next step, we will continue to research the influence of the amount and type of alkali activator on the pore structure of foamed concrete materials and the amount of carbon dioxide adsorption.

Conclusions
In this paper, foam concrete materials were prepared from cement and solid wastebased materials using hydrogen peroxide as the foaming agent. The effects of water-cement ratio and foaming agent dosage on CO 2 adsorption were studied by XRD, BET, SEM, and TGA. The following conclusions have been drawn.
(1) XRD analysis reveals that the hydration products of the cementitious material mainly include C-S-H, Ht, and Ca(OH) 2 , and they are responsible for CO 2 sequestration. The types of hydration products remain the same, despite the changes in water-cement ratio and foaming agent dosage.
(2) BET analysis suggests that the pores in the foam concrete materials are mainly mesopores. The specific surface area, pore volume, and average pore size of the material change with the changes in water-cement ratio and foaming agent dosage. When the water-cement ratio, foaming, and dosage increase continuously, the specific surface area of foamed concrete materials increases first and then decreases. When the water-cement ratio is 0.7, the specific surface area reaches the maximum, increasing the contact area between CO 2 and materials and increasing the adsorption point, which is conducive to the solidification of CO 2 .
At the water-cement ratio of 0.7 and the foaming agent dose of 10%, the obtained foam concrete exhibits a specific surface area of 25.412 m 2 /g, a pore volume of 0.144 cm 3 /g, and an average pore diameter of 12.840 nm.
(3) SEM analysis shows that the hydration degree of the cementitious material is low at low water-cement ratios and low foaming agent dosages. The surface structure of the obtained sample is dense, with fewer pores and ununiform big pores. At the water-cement ratio of 0.7 and foaming agent dose of 10%, more pores with uniform sizes are formed, and even connected pores are observed, which is conducive to the CO 2 diffusion in the material.
(4) CO 2 sequestration can be determined from the TGA and DTG curves. The decompositions of the hydration products or the carbonate produced from carbonization of Ca(OH) 2 occur in the temperature range of 550-850 • C. CO 2 sequestration first increases and then decreases with the increases of water-cement ratio and foaming agent dosage. The highest CO 2 sequestration of 66.35 kg/m 3 is obtained at the water-cement ratio of 0.7 and the foaming agent dose of 10%.
Through the CO 2 curing experiment of foamed concrete materials prepared under the conditions of water-cement ratio and foaming agent, it is concluded that when the water-cement ratio is 0.7 and foaming agent dosage is 10%, the foamed concrete materials have uniform pore size, high porosity and connected pore ratio, large specific surface area of pores, and the highest CO 2 sealing stock is 66.35 kg/m 3 . Therefore, the water-cement ratio is 0.7. The foaming agent content of 10% is the best proportion in this group of raw material systems.